i already download the Firefox, why does it still say to download Firefox?? I've tried to download the cookie editor file ibut it didn't work.

What kind of device/operating system is this?

Is this with Firefox on iPhone/iPad (iOS/iPadOS) if so then there are no Extensions. https://support.mozilla.org/en-US/kb/add-ons-firefox-ios The Extensions are only available for Firefox on Android and desktop Firefox on Windows, macOS, Linux.